Computing devices, such as notebook computers, personal data assistants (PDAs), mobile handsets and the like, all have user interface devices. One class of user interface device that has become more common is based on capacitive touch-sensor technology utilizing touch-sensitive capacitors. Touch-sensitive capacitors may be used to implement touch-sensor pads, such as the familiar mouse pad in notebook computers, non-mechanical slider controls (e.g., a volume control) and non-mechanical push-button controls.
FIG. 1A illustrates a typical touch-sensor pad 100. The touch-sensor pad 100 includes a sensing surface 101 on which a conductive object may be used to position a cursor in the x- and y-axes, or to select an item on a display. Touch-sensor pad 100 may also include two buttons, left and right buttons 102 and 103, respectively, which may operate as touch-sensitive switches.
FIG. 1B illustrates a conventional linear touch-sensor slider (“slider”) that might be used as a linear control such as a volume control, for example. The slider 110 includes a number of conductive sensor elements 111 separated by insulating gaps 112, where each sensor element is an electrode of a capacitor. Typically, a dielectric material (not shown) is overlaid on top of the sensor elements to prevent any direct electrical conduction between the sensor elements and/or a conductive object when the conductive object is placed on the slider. When a conductive object contacts or comes in proximity to one of the sensor elements, a capacitance associated with the sensor element (or with an adjacent pair of sensor elements) is changed. The change in capacitance can be detected and sent as a signal to a processing device. As a finger or other conductive object moves across the slider, the changing capacitance of each sensor element is detected to pinpoint the location and motion of the conductive object. This same principle (i.e., detecting capacitance changes) can also be used to implement touch sensor buttons (e.g., on-off controls).
FIG. 2A illustrates one form of a touch sensitive capacitor 300. In its basic form, the touch sensitive capacitor 300 includes a pair of adjacent plates 301 and 302. There is a small edge-to-edge (fringing) capacitance Cf between the plates. When a conductive object 303 (e.g., a finger) is placed in proximity to the two plates 301 and 302, there is a capacitance between the conductive object and each of the plates. If the capacitance between the conductive object and each plate is defined as 2*CS, then the total capacitance between the plates due to the presence of the conductive object is CS (the series combination of the two separate capacitances). This capacitance adds in parallel to the fringing capacitance Cf between the plates 301 and 302, resulting in a change in total capacitance equal to CS.
FIG. 2B illustrates another form of a touch sensitive capacitor 307 where two parallel plates 305 are separated by a dielectric layer 308 and one of the plates is grounded. Typically, the ungrounded plate is covered by a second dielectric layer 304. The parallel plate capacitance between the two plates 305 is denoted by Cpp. When the conductive object 303 approaches or contacts dielectric layer 304, a capacitance CS is created between the conductive object and the ungrounded plate. As a result, the total capacitance from the ungrounded plate to ground is given by the sum of the capacitances Cpp+Cs (the conductive object need not be actually grounded for the touch sensitive capacitor to operate; a human finger, for example, is connected to a person's body capacitance, which can act as a virtual ground). Detecting a touch is then a matter of measuring the change in capacitance from Cpp to (Cpp+Cs). In a typical touch sensitive capacitor, Cs may range from approximately 10 to 30 picofarads (pF), although other ranges may be used. While the conductive object illustrated here is a finger, any conductive object may be used (e.g., a stylus).
A variety of different circuits have been developed that can be used to detect and/or measure the capacitance and/or capacitance changes of touch sensitive capacitors. One type of detection circuit, known as a relaxation oscillator, uses the varying capacitance of the touch sensitive capacitor to control the frequency of oscillation of the relaxation oscillator. When the capacitance of the touch sensitive capacitor changes due to the proximity or contact of a conductive object, a corresponding change in the frequency of the oscillator signals the capacitance change and can be used to locate the position of the conductive object (in the case of a pad or slider, for example) or to trigger the performance of some function related to the touch-sensor. A conventional relaxation oscillator develops a voltage across a touch sensitive capacitor by charging the capacitance of the touch sensitive capacitor with a current source, from ground potential to a threshold voltage, and when the voltage reaches the threshold voltage, the touch sensitive capacitance is discharged to ground and the charging process begins anew. The time required for the voltage to increase from the ground potential to the threshold voltage is the period (reciprocal of frequency) of the oscillator and provides an indirect measure of the capacitance. If the capacitance changes (e.g., due to the proximity of a conductive object), the period (and frequency) of the oscillator changes.
FIG. 3A illustrates a conventional capacitance sensing relaxation oscillator 350. In FIG. 3A, a current source 355 drives a constant current I through a sense capacitor 351. The constant current I charges the capacitor and causes the capacitor voltage VC to increase linearly. When VC exceeds a threshold voltage VTH, the output 353 (VOUT) of comparator 352 goes high and closes switch 354 to discharge the sense capacitor 351 to ground. The output 353 of the comparator 352 goes low, opening switch 354, and the cycle starts over.
One disadvantage of conventional relaxation oscillators, or any oscillator that discharges a frequency control element (such as a capacitor) to ground, is that the ground reference is generally noisy (in an electrical sense), especially in digital signal environments. Ground noise causes uncertainty in the starting voltage of the touch sensitive capacitor after discharge, which results in uncertainty (i.e., jitter) in the period of oscillation. As a result, the precision of the capacitance sensing/measurement is impaired. If the noise is not random (as in the case of digital noise, for example), the uncertainty cannot be removed by averaging.
This effect is shown in FIG. 3B, which illustrates the capacitor voltage VC versus time in the circuit of FIG. 3A. In FIG. 3B, the solid waveform 356 represents an ideal relaxation oscillator waveform that would be produced by a noiseless ground. Box 357 represents the range of uncertainty in the ground potential due to noise. This uncertainty is translated to uncertainty in the timing of waveform 356, represented by the dotted waveforms 358 and 359, which are shown bracketing the ideal waveform 356, which has a nominal period of oscillation T (the uncertainty applies to every period, even though not shown in FIG. 3B). FIG. 3C illustrates how the uncertainty in the timing of VC affects the output 353 (VOUT) of the relaxation oscillator 350. In FIG. 3C, the solid waveform 360 represents an ideal output waveform that would be produced in the absence of ground noise, while the dotted waveforms 361 and 362 illustrate the range of jitter in the output introduced by the ground noise 357. The actual frequency of oscillation of the relaxation oscillator will then jitter around the nominal frequency fOSC from period to period.